This invention relates generally to techniques for non-destructively testing fabricated metal components and, more particularly, to eddy current inspection systems.
The eddy current method of non-destructively evaluating metal products is widely used as many industries today. The theory and application of this method are described in detail in the "Non-destructive Testing Handbook", R. C. McMaster, editor, Ronald Press, New York, sections 36-38 and 40, 1959.
In the nuclear power industry, however, eddy current inspection has heretofore been unable to test for flaws and incipient defects in the dented regions of steam generator tubes.
Referring to FIGS. 2 and 3, reference numeral 10 indicates the dented region of a typical steam generator tube. The dented region includes the "edge of the dent" 12 which is a zone of transition from the normal tube diameter to the reduced diameter. The edge of the dent occurs near the corners of the tube support 7. The dented region also includes a wrinkle 14 which is a slightly raised area that appears circumferentially around the outside wall of the tube. The wrinkle occurs near the center of the tube support region and can be accompanied by a slightly depressed area on the inner tube wall directly beneath the raised area indicated by reference number 14. From FIG. 2 it can also be seen that denting results in a non-uniform reduction in the diameter of the tube and produces either eliptical or non-circular tubes.
Steam generator tube denting is currently believed to the result of an accumulation of materials 6 between the tube support 7 and the tube 8. These materials are the corrosion products of the accelerated corrosion of the carbon steel support plate 7. During the reactor start up thermal expansion causes a swaging action on the tube which in turn causes a reduction in the diameter of the tube without apparently changing the thickness of the tube wall.
The problem of denting is a very serious concern in the nuclear power industry because the dented regions of steam generator tubes presently cannot be inspected for flaws and incipient defects. Such defects can cause a tube to rupture and result in both a loss of primary coolant from the reactor and the radioactive contamination of the secondary steam system. Cracks and pits are also difficult to detect when located at the edge of a dent 12, beneath a wrinkle 14, near the corner of a tube support 7, or in areas of crud buildup 6. Tube wall thinning or wastage is also difficult to characterize accurately in dented regions.
The currently available eddy current probes used for steam generator tubing inspections cannot adequately characterize the cross-sectional profile of the dented region because of the undesired signals from the tube supports, the variations in diameter, ovality, the build up of curd between the support and the tube and the presence of wrinkles. Currently available eddy current probes are particularly sensitive to variations in tube diameter in the dented region. In addition, severe diameter restrictions and ovality can restrict the passage of standard diameter probes. Further, the signal from a dent can be from twenty to fifty times larger than typical flaw signals. These large signals prevent any characterization of a flaw because the signal processing network is over-driven.
The method currently used for inspecting dents with eddy currents employs a small rotating point coil operating in tandem with a differential encircling coil probe. The differential encircling coil operates in the conventional manner and measures the tube for stress and wastage corrosion. If denting is observed in the tube by the differential coil probe, the dented area is then helically scanned using the small direct contact coil. Such helical scanning is a time consuming process, produces data that is difficult to interpret and extends the time that the nuclear reactor must be shut down. In addition, the electrical signal output from the point coil must be commutated by slip-rings which add to the cost and the noise in the system.